There are two potential legal issues.
1. Did you sign a non-compete clause as part of your relationship with the previous employer?
2. When attempting to negotiate new contracts as a self business, to what degree are you going to "pirate" the estimating process from the previous employer?
Both of these are serious issues that only you can address/answer.
Regarding the moral/ethical aspect, this all depends on the type of contract each client has with the previous employer. Yearly, per service, quarterly, what?
As far as "stealing" clients from a previous employer, as a successful business owner myself I view it this way: Provided I am capably caring for the client I am not worried a previous employee will "steal" from my client base.
On numerous instances clients have directly contacted me after being approached yet did not change due to my providing a level of service the client appreciated/and paid for.
I will say, and karma has nothing to do with this at all, provided you are able to woo clients away be prepared for their now being "trained" that it is okay to price shop = you may very well be the victim of what you are trying to initiate.
Business is business, simply persevere in providing good service and if you have the guts to hang in through the growing pains then you might be alright.
Nice Try = You Suck Spelled Differently